Spinel is a magnesium aluminum oxide that forms bright, hard crystals in red, pink, orange, purple, blue, gray and black. For centuries, red spinel was treated as ruby, and several of the most famous "rubies" in European royal collections are spinels. Spinel is rarely treated, ranks 8 on the Mohs scale and became an official August birthstone in 2016.
What spinel is
Spinel is both a mineral species and the name of a mineral group. The gem species has the formula MgAl2O4. It crystallizes in the cubic system, typically as sharp octahedrons, and twins called spinel-law twins are common enough that the term is used for the same twinning in diamond and other minerals. The page on crystal forms and twinning shows these shapes.
Because spinel is cubic, it is singly refractive. Light passes through it at one speed in every direction, so it shows no pleochroism. That separates it from ruby, which is doubly refractive and dichroic. Spinel's refractive index usually sits near 1.718, and its specific gravity near 3.60, both lower than corundum.
Colors and their causes
- Red and pink spinel owe their color to chromium. Strong red stones fluoresce under ultraviolet light.
- Orange to orangy pink stones, sometimes called flame spinel, combine chromium with other elements.
- Purple, lavender and gray spinels are colored by iron, sometimes with chromium.
- Blue spinel is usually colored by iron. A rare, vivid cobalt blue spinel, found mainly in Vietnam and Sri Lanka, owes its color to cobalt.
- Black spinel is opaque and contains abundant iron.
Famous stones that turned out to be spinel
Until the late 18th century, gem identification relied mainly on color, so red spinel and ruby were grouped together. Mineralogists separated spinel from corundum as a distinct species in the late 1700s, and later testing revealed several famous spinels in royal treasuries.
The Black Prince's Ruby is a large red spinel, roughly 170 carats, set at the front of Britain's Imperial State Crown. Tradition links it to Edward of Woodstock, the Black Prince, in the 14th century, and Henry V is said to have worn it at Agincourt in 1415. It is still called a ruby by name.
The Timur Ruby, a red spinel of more than 350 carats in the British Royal Collection, is engraved with the names of several earlier owners, including Mughal emperors. Its association with Timur comes from tradition more than proof. The large red stone on the Russian imperial crown of Catherine the Great is also a spinel.
Much historic red spinel came from mines in Badakhshan, in present-day Tajikistan and Afghanistan. Medieval writers called it balas ruby after the region. The page on Persian jewelry and the page on Indian jewelry history describe the courts that prized these stones.
Where spinel comes from
Spinel forms mostly in marble, created when impure limestone is changed by heat and pressure, the same setting that produces fine ruby. It also occurs in alluvial gravels where weathering frees the hard crystals. The page on metamorphic rocks and gemstones explains these deposits.
Myanmar
The Mogok area is the classic source of red and pink spinel, and the Namya area in northern Myanmar has produced bright pink and red stones. Mogok spinels often occur alongside ruby in the same gravels. See Mogok ruby for the geology.
Tanzania
Deposits near Mahenge in southern Tanzania gained wide attention in 2007, when miners found large crystals of intense, glowing pink to red spinel. Mahenge spinel helped raise the gem's profile in the trade, and Tanzania also produces spinel in other colors from Tunduru and elsewhere.
Sri Lanka, Vietnam and others
Sri Lanka's gem gravels produce spinel in many colors, including pastel pinks, purples and cobalt blue. Vietnam, particularly the Luc Yen area, yields red, pink and purple spinel and some cobalt blue stones. Tajikistan, Afghanistan, Pakistan and Madagascar also produce gem spinel.
Treatments
Most spinel is sold untreated, which appeals to buyers who want a natural stone. Some stones are heated, and labs can sometimes detect heating with photoluminescence spectroscopy and microscopic study. Fracture filling occurs in lower-grade goods. Any treatment should be disclosed. The overview of gemstone treatments compares spinel with gems that are routinely treated.
Synthetic spinel and imitations
Synthetic spinel has been made by flame fusion since the early 20th century. Manufacturers produced it in colors that imitate aquamarine, blue zircon, peridot and other gems, and many class rings and older costume pieces contain synthetic spinel. Flame fusion spinel often shows a strain pattern in a polariscope (known as anomalous double refraction) and has a slightly higher refractive index than most natural spinel. Flux-grown synthetic red and pink spinel can be much harder to identify. Labs use inclusions and spectroscopy to separate it from natural stones.
Judging spinel quality
Color is the main factor. Saturated red, hot pink, orange-pink and cobalt blue stones command the highest prices. Spinel tends to be cleaner than ruby, and many stones are eye-clean, so clarity standards are higher. Octahedral inclusions of small spinel crystals, sometimes in rows, are characteristic. Cut affects brightness, and well-cut spinel has a lively sparkle. Large, fine red spinels are rare. See how colored gemstones are graded.
Care and durability
At 8 on the Mohs hardness scale, spinel resists scratching from everyday dust and has no easy cleavage, so it suits rings worn daily. Clean it with warm soapy water. Ultrasonic and steam cleaning are usually safe for untreated stones without fractures.
August birthstone since 2016
In 2016, the American Gem Trade Association and Jewelers of America added spinel to the list of August birthstones, alongside peridot and sardonyx. The change gave August buyers more choice of color, since spinel comes in many hues.
Frequently asked questions
How do you tell spinel from ruby?
Spinel is singly refractive and shows no pleochroism, while ruby is doubly refractive and dichroic. A refractometer, polariscope or dichroscope separates them quickly.
Is the Black Prince's Ruby a ruby?
No. It is a large red spinel. It kept its historic name after spinel was identified as a separate mineral from corundum.
Is spinel usually treated?
Most spinel is sold untreated. Some stones are heated or have filled fractures, which should be disclosed, but treatment is far less common than in ruby or sapphire.
Why is synthetic spinel so common in old jewelry?
Flame fusion synthetic spinel was cheap to make in many colors during the 20th century, so manufacturers used it in class rings, birthstone jewelry and costume pieces.